It is known that the auditory apparatus may be affected by a variety of malfunctions. Where there is diminished sensitivity we have cases of perceptive hypacusis, while when the middle ear is damaged, either for congenital reasons or as a consequence of chronic infections or trauma, or as a result of surgical causes, such as surgery for radical or modified radical mastoidectomy, we then refer to transmissive and/or mixed hypacusis.
Many acoustic prostheses are currently available to overcome the abovementioned conditions. Among these mention may be made of those which have a direct action on the middle ear. These prostheses having a direct action on the middle ear generally comprise a microphone, possibly means for processing the auditory signal, a loudspeaker or acoustic transducer and a waveguide capable of directly reaching the oval window, bypassing the tympanic membrane and ossicles. In these acoustic prostheses the electronic units (microphone, possible signal processing means, loudspeaker or acoustic transducer) are placed in the auditory canal and are connected to powering batteries by electrical wires.
These types of acoustic prostheses give rise to various problems for users when they have to be removed, if only temporarily. Users in fact often have to be able to remove the prosthesis from their ear, for example for some sports activities such as swimming.
Acoustic prostheses also have to be removed for medical reasons. In fact it is known that metal objects cannot be worn when examinations such as CAT (computerized axial tomography) of the brain or magnetic resonance are performed. Thus, it is desirable that the prosthesis in question should be capable of being easily removed, by another person such as a doctor or nurse or the patient himself, so that the patient can undergo urgent diagnostic procedures.
At the present time some phenomena which have been revealed by investigations into the neurophysiopathology of auditory prostheses, such as:                secondary adaptability, acclimatization;        deprivation effects described by various researchers are becoming increasingly important.        
It is known that the central nervous system (CNS) must rehabituate itself to the reintroduction of sounds which have disappeared or become attenuated, when they are restored to the auditory circuit following amplification by the prosthesis. In cases of perceptive deafness there is a reduction in the auditory input from the periphery to the central nervous system and either adaptation, which follows a period of acclimatization, or a negative reaction may take place.
As for all the various areas of the human body, including the acoustic areas, there are maps in the cerebral cortex which correspond to neurons originating from the zone of the basal membrane (Corti's organ). Low frequencies are located at the apex of the cochlea and high frequencies at the base.
In the case of unilateral perceptive hypacusis to high frequencies there is a change in the cells of Corti's organ, a thinning or degeneration of the myelin of the nervous fibers, while other cells or fibers remain intact.
Thus, with the passage of time, owing to the lack of stimuli from the periphery to the center, changes take place in the central nervous system corresponding to the areas of projection.
The reduction or absence of inhibiting inputs which should reduce the spontaneous activity of cells in the central nervous system gives rise to significant changes in the central nervous system itself. It is thus understandable that reintroduced amplified sounds can find themselves in an environment which is often greatly changed as a result of anatomical and physiological changes such as for example:                thinning of the myelin of neurons and synapses, and/or        changes in spontaneous activity and inhibition.        
The introduction of amplified sounds thus gives rise to an appreciable reaction in a system which had already adapted itself to a previous situation, even though a pathological one, and therefore the period of re-adaptation can last for a variably long time.
To sum up, with an acoustic prosthesis there is brought about reorganization of the map of auditory frequencies, “reusing” neurons which no longer respond to the stimuli applied to a damaged cochlear region. This therefore contributes to the hearing of some frequencies which can still be heard. Thus, if frequencies which are no longer heard are now restored through the acoustic prosthesis, competition for their old neurons is set up, thus giving rise to an imbalance in the codification of the sound maps.
The phenomenon of reversibility thus becomes a very important factor. If, however, secondary re-adaptability cannot be brought about with the prosthesis, it is generally better not to keep trying.
The above shows how much more difficult it can be for the adaptability brought about by a prosthesis in a patient with reduced bilateral perceptive hypacusis, when an effort is made to rehabilitate the maps of debilitated high frequency neurons while at the same time also acting on low frequency neurons that are still perfectly functional.
After careful clinical investigations into patients in whom prostheses according to the known art have been implanted, in particular the prosthesis described in patent IT 1294267, of which the Applicants are the proprietor, in which the prosthesis provided for an amplifier located within the auditory canal together with the microphone, these phenomena of secondary adaptability and acclimatization were still present, even though minimally, in 25% to 27% of patients treated.
Thus it was established that in order to overcome the limitations of the prostheses according to the known art it was preferable to reduce occupation of the canal as much as possible, while at the same time maintaining as direct an action as possible on the middle ear.
A further technical problem with acoustic prostheses according to the known art is that of anchoring the extremity of the waveguide. Securing this component makes it possible to stabilize the action of conveying sound to the nerve cells of the middle ear, improving the user's response to the phenomena of secondary adaptability and acclimatization.
At the present time metal hooks attached to the malleus are used to anchor the waveguide. This solution presents various problems. In fact these hooks:                are difficult to attach securely to the extremity of the waveguide located within the middle ear;        can become detached;        are made of metal, which means that the patient will not be able to undergo instrumental examinations such as cranial CAT or magnetic resonance for the reasons mentioned above; and        damage the malleus in the long term.        